This invention relates to a reference current source circuit.
There is conventionally known an integrated reference current source circuit which comprises a first current source (current sink) including first and second NPN-type transistors having different emitter areas and bases connected with each other, one of the transistors being diode-connected, and a resistor connected to the emitter of one of the transistors having a greater emitter area, so that the currents (the collector currents of the first and second transistors) of the same magnitude which is dependent on the emitter area ratio between the first and second transistors and the resistance value of the resistor are caused to flow into the first current source, a second current source or current mirror using PNP-type transistors, whereby the collector currents of the first and second transistors of the same magnitude are allowed to flow into the first current source, and an NPN-type output transistor driven by the first current source to provide an output current. For this circuit, refer to FIG. 10 on page 7 of a paper entitled "Integrated linear basic circuits" (Th. J. van Kessel and R. J. van de Plassche, Philips Technical Review, Vol. 32, 1971, No. 1, pp. 1-12).
The aforesaid output transistor is connected with the diode-connected transistor of the first current source in current-mirror configuration. If the diode-connected transistor and output transistor have the same emitter area, the output current I.sub.0 is equivalent to the collector currents of the first and second transistors, and is given by ##EQU1## where V.sub.T is the volt-equivalent of temperature, R is the resistance value of the emitter resistor, and N is the ratio between the emitter areas of the first and second transistors. Evidently, the output current I.sub.0 is independent of supply voltage and is proportional to temperature.
In this prior art reference current source circuit, a current mirror comprised of PNP transistors is used as the second current source in order that the sink currents of the same magnitude are produced by the first current source. As is generally known, a PNP transistor has smaller .beta. (common-emitter current gain) than an NPN transistor, so that base current of the PNP transistor cannot be ignored. As compared with an NPN-transistor current mirror, therefore, the PNP-transistor current mirror is subject to greater error, with respect to the ideal value "1", in the ratio between the magnitudes of two currents flowing through the current mirror. Due to such error, the output current is also subject to an error with respect to a desired value. Where a Wilson source comprised of three PNP transistors is used as a current mirror, as shown in FIG. 10 in the aforementioned paper, the current ratio of the current mirror further approaches 1, though it still is subject to the influence of .beta.. Owing to the temperature-dependence of .beta. of PNP transistors, moreover, the current ratio of the current mirror is liable to change with temperature. Thus, with use of a current mirror including PNP transistors, it is relatively difficult to allow two currents of the same magnitude dependent on the emitter area ratio between the first and second transistors and the emitter resistor to flow into the first current source. In the aforementioned circuit, furthermore, there is required a start-up circuit for actuating the circuit when a power is applied to the circuit.